1. Field of the Invention
The invention relates to use of catalysts to enhance yields of olefins and liquefied petroleum gas (LPG) produced in a fluidized catalytic cracking (FCC) process.
2. Description of Related Art
A discussion relating to use of ZSM-5-based catalysts to enhance olefin yields in FCC processes is found in U.S. Pat. No. 5,997,728. The following description of related art is based on that discussion.
Catalysts used in FCC processes are in particle form, usually have an average particle size in the range of 20 to 200 microns, and circulate between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts hot, regenerated catalyst which vaporizes and cracks the feed at about 400° C. to 700° C., usually 500° C. to about 550° C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating it. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air, to restore catalyst activity and heat catalyst to, e.g., 500° C. to 900° C., usually 600° C. to 750° C. The hot regenerated catalyst recycles to the cracking reactor to crack more fresh feed. Flue gas from the regenerator may be treated to remove particulates or convert CO, and then discharged into the atmosphere. The FCC process, and its development, is described in the Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, in the Jan. 8, 1990 edition of the Oil & Gas Journal.
The product distribution from current FCC processes comprises a number of constituents, with gasoline being of primary interest to most refiners. Light olefins and LPG are also found in the FCC product, and are increasingly becoming of interest to refiners as those products become more valuable. The light olefins produced can be used for a number of purposes, e.g., they are upgraded via sulfuric or HF alkylation to high quality alkylate. LPG is used for cooking and/or heating purposes. Accordingly, operators of FCC units can vary the content of their products depending upon the markets they are serving and the value associated with each of the components found in an FCC product.
Propylene is a particular light olefin in high demand. It is used in many of the world's largest and fastest growing synthetic materials and thermoplastics. Refiners are relying more and more on their FCC units to meet the increased demand for propylene, thus shifting the focus of the traditional FCC unit away from transportation fuels and more toward petrochemical feedstock production as operators seek opportunities to maximize margins.
If a refinery cannot expand its existing unit, FCC operators have rather limited options for increasing light olefin production. Reported options include:                a. FCC processes employing ZSM-5 and large pore zeolite that share matrix, i.e., an integral catalyst.        b. FCC processes using additive ZSM-5 catalyst.        c. Production of cracked gas from gas oil over pentasil zeolites at high cracking severity.        
These approaches are reviewed in more detail below.
Integral Catalysts Containing Large Pore Zeolite Catalyst+ZSM-5
U.S. Pat. No. 3,758,403 discloses adding ZSM-5 to conventional large pore zeolite cracking catalyst formulations, including adding ZSM-5 during manufacture of the large pore zeolite catalyst particles so that the ZSM-5 is integrated into the catalyst particle. Based on '403, use of large pore zeolite cracking catalyst containing large amounts of ZSM-5 additive that has been integrated into the catalyst gives only modest increases in light olefin production. A 100% increase in ZSM-5 content (from 5 wt % ZSM-5 to 10 wt % ZSM-5) increased the propylene yield less than 20%, and decreased slightly the potential gasoline yield (C5+gasoline plus alkylate).
U.S. Pat. No. 6,566,293 discloses another type of integral catalyst wherein phosphorus is combined with the ZSM-5 and calcined prior to their addition to matrix, and optionally, and in certain instances, preferably large pore zeolite Y. The resulting slurry of calcined ZSM-5/phosphorus and matrix-containing slurry is then spray dried into catalyst. The '293 patent reports that these catalysts are efficient in olefins production, while also maintaining bottoms cracking. See also “FCC Meets Future Needs”, Hydrocarbon Engineering, January 2003.
ZSM-5 Additives
Refiners have also been adding ZSM-5 containing catalysts as additive catalysts to their FCC units, with 10-50 wt %, more usually 12 to 25 wt %, ZSM-5 in an amorphous support. In this instance, the ZSM-5 is added as particles that are separate from the particles containing the conventional large pore zeolite catalysts. ZSM-5 has been primarily added to FCC units for gasoline octane enhancement, but as mentioned above, it is also used to enhance light olefins. Such additives have physical properties that allow them to circulate with the large pore zeolite cracking catalyst. Using ZSM-5 in a separate additive allows a refiner to retain the ability to use the myriad types of commercially available large pore zeolite cracking catalyst available today.
U.S. Pat. No. 4,309,280 discloses adding very small amounts of powdered, neat ZSM-5 catalyst, characterized by a particle size below 5 microns. Adding as little as 0.25 wt % ZSM-5 powder to the FCC catalyst inventory increased LPG production by 50%. Small amounts of neat powder behaved much like larger amounts of ZSM-5 disposed in larger particles.
A method of adding a modest amount of ZSM-5 to an FCC unit is disclosed in U.S. Pat. No. 4,994,424. ZSM-5 additive is added to the equilibrium catalyst in a programmed manner so an immediate boost in octane number, typically ½-2 octane-number, is achieved.
U.S. Pat. No. 4,927,523, discloses adding large amounts of ZSM-5 to a unit without exceeding wet gas compressor limits. Large amounts were added, and cracking severity reduced until the ZSM-5 activity tempered from circulating through the FCC unit for several days.
Development on ZSM-5 additives has also been directed at stabilizing them with phosphorus or making the additives more attrition resistant. Phosphorus stabilized ZSM-5 additive is believed to retain activity for a longer period of time, thereby reducing the makeup rate of ZSM-5 additive required. Even with phosphorus stabilization, refiners interested in maintaining gasoline yield fear dilution of the large pore zeolite cracking catalyst by addition of ZSM-5, e.g., over 2 or 3 wt % ZSM-5 crystal. Use of more than 5 or 10 wt % additive will reduce yields of gasoline and seriously impair conversion. Most refiners therefore are still faced with using ZSM-5 additives at amounts significantly smaller than the upper limits recited above.
Moreover, the aforementioned Hydrocarbon Engineering article highlights that adding more ZSM-5-based additives, even those that are stabilized by phosphorus, has diminishing returns, because more Y zeolite is usually added to reduce cracking catalyst dilution caused by the additional amount of ZSM-5. The addition of more zeolite Y in turn increases hydrogen transfer to the molecules that ZSM-5 converts into olefins. The net effect of increasing the Y zeolite is reduced light olefins because the olefins saturated by the hydrogen are not available for conversion by ZSM-5 into light olefins. As a result the authors suggest adopting a new embodiment of integral catalyst such as that described above.
Based on the experience embodied in the aforementioned patents, ZSM-5 additive has been recognized as a way to increase C3 and C4 olefin yields and gasoline octane. It, however, is used at the cost of loss in gasoline yield. It is therefore submitted that based on the understanding in the art ZSM-5 would be of most benefit to refiners when used in small amounts, preferably in FCC units operating at modest severity levels.
The art has also recognized that olefins yields from FCC processes can be affected by rare earth content of Y zeolite-based catalysts containing relatively low level of ZSM-5-based olefins additives. See “ZSM-5 Additive in Fluid Catalytic Cracking II, Effect of Hydrogen Transfer Characteristics of the Base Cracking Catalysts and Feedstocks”, Zhao et al., Ind. Eng. Chem. Res., Vol. 38, pp. 3854-3859 (1999). For example, rare earth is widely used in zeolite Y-based catalysts to increase activity and conversion of the feedstock into FCC products. These exchanged zeolites are then formulated with matrix and binder to form the finished catalyst compositions, or further blended with ZSM-5 to form the final catalyst additive. Typical REY-based catalyst contains about 2% by weight rare earth, which usually equates to the Y zeolite containing about 5% by weight, based on the zeolite. Zhao et al. have found, however, that using REY having 2% by weight rare earth reduces olefins yields when compared to Y zeolites that contain smaller amounts of RE, including no RE at all. It is believed RE exchanged Y zeolite increases hydrogen transfer reactions, which in turn leads to saturation of olefins in the gasoline range. As indicated earlier, olefin molecules in the gasoline range can be converted into propylene and butylenes, and their saturation removes molecules that could be converted into light olefins. Accordingly, there is a suggestion in the art that one could enhance olefin yields by reducing rare earth content when formulating catalyst containing RE exchanged zeolites and ZSM-5.
High Severity Conversion Using Pentasil
U.S. Pat. No. 4,980,053, describes examples of converting vacuum gas oil to more than 50 wt % cracked gas over zeolites ranging from pentasil to USY, and mixtures thereof. The process is basically a pyrolysis process, which uses a catalyst to operate at somewhat milder conditions than thermal pyrolysis processes.
The catalysts A-D described in the '053 patent were used in a process run at conditions much more severe than those used in typical catalytic cracking—580° C. (1076° F.), at a 1 LHSV, a cat:oil ratio of 5, and steam:hydrocarbon ratio of 0.3.
Catalystwt % of:ABCDcracked gas52.051.254.055.6propylene11.6117.3921.5621.61butylene15.6414.4715.6415.09C5-205 C31.033.127.027.5Conversion93.390.387.689.1
While the zeolite content of the catalysts is not specified, the patentees of the '053 patent report that “the yields of gaseous olefins over catalyst C (Pentasil) and D (D=mix of pentasil+USY) are higher than the others.” As far as gasoline yields, and conversion, the mixture in D gives less conversion and less gasoline yield than a single particle catalyst (A=Pentasil+REY). Use of a mixture also reduced butylenes yields slightly, as compared to single particle catalyst A. Catalyst B is reported to be a USY-type zeolite catalyst.
Example 2 of '053 reports production of fairly aromatic gasolines, containing more than 50 wt % aromatics. This was to be expected from the high temperatures and severe conditions. The octane number of the gasoline was 84.6 (motor method). The di-olefin content of the gasoline was not reported.
These results show use of separate additives of pentasil zeolite can reduce conversion and butylene and gasoline yield, as compared to use of single particle catalyst with both types of zeolite in a common matrix, during a pyrolysis process.
As a solution to the various problems mentioned above, U.S. Pat. No. 5,997,728 discloses a catalytic cracking process for converting a heavy hydrocarbon feed to lighter products comprising; charging a heavy hydrocarbon feed comprising hydrocarbons boiling above 650° F. to a riser catalytic cracking reactor; charging a hot fluidized solids mixture, from a catalyst regenerator to the base of said riser reactor, said mixture comprising: a physical mixture of regenerated base FCC cracking catalyst and separate particles of shape selective zeolite cracking catalyst additive, said mixture containing 87.5 to 65 wt % base FCC catalyst and 12.5 to 35 wt % additive, and wherein said additive comprises a catalytically effective amount of a zeolite having a silica:alumina ratio above 12 and a Constraint Index of 1-12 (e.g. ZSM-5) in an amorphous support. The feed is catalytically cracked at conditions including a riser outlet temperature of about 925 to 1050° F. to produce catalytically cracked products including ethylene, propylene, and a C5+ gasoline fraction. The cracked product from this process is said to produce after fractionation at least 44.0 wt % C5+ 15 LV % propylene, (i.e., about 9% by weight propylene), and no more than 2.0 wt % ethylene.